A common practice in many medical diagnostic procedures is to penetrate into some portion of a living mammal with a hollow needle or cannula through which tissue or fluid can be withdrawn. Amniocentesis is an example of such a procedure.
Amniocentesis was first suggested more than 90 years ago. Following up on this suggestion, physicians were able to relieve an overdistended uterus by tapping some of the amniotic fluid. Still later, it was discovered that amniotic fluid could be used for diagnostic purposes, e.g. in studies of Rh-sensitized women, assessment of fetal maturity, antenatal diagnosis of genetic disorders (including chromosomal abnormalities, inborn errors of metabolism, congenital malformations, etc.), determination of fetal sex, and the like. The procedure has been used particularly in the second and third trimesters of pregnancy. In the second trimester, it is typically carried out for the diagnosis of genetic disorders and to determine the sex of the fetus, whereas in the third trimester, fetal maturity studies and Rh factor determination are perhaps the most common indications for the procedure.
Transabdominal amniocentesis is currently carried out with the aid of a standard 19 or 20 gauge, 7-10 cm spinal needle which consists of a stylus fitted into a hollow cannula. Selection of the appropriate site for amniocentesis depends primarily on determination of the fetal lie by manual palpatation of the abdomen, and is further facilitated by an ultrasound examination for localization of the placenta and an accessible pocket of fluid. The spinal needle is advanced through the different layers of the abdomen, through the muscular wall of the uterus, and entry into the amniotic sac is typically recognized by the sudden loss of resistance to the forward progress of the needle. The stylus is then withdrawn, a syringe attached to the hub of the needle, and amniotic fluid is aspirated.
Despite the simplicity of the procedure and the increasing use of ultrasonics to facilitate the localization of the placenta, failures as well as complications to the fetus continue to occur as a result of the procedure. Literature has appeared which suggests the possibility of a number of minor as well as very serious complications. For example, see Creasman et al, "Fetal Complications of Amniocentesis", JAMA, 204:91 (1968), and Robinson et al, Am. J. Obstst. Gynecol., 116:937 (1973).
One of the inconveniences of the procedure occurs in the case of a "dry tap". For reasons which are not clearly understood, it occasionally happens that fluid cannot be aspirated, even though the distal end of the needle is inside the amniotic sac. Unfortunately, it can be difficult to distinguish a dry tap from a situation in which the needle has not yet penetrated the sac. A dry tap indicates that another puncture site should be selected, while a failure to penetrate the sac indicates that advancement of the needle should be continued. For both patient and practitioner, it would be a great convenience if dry taps could be recognized quickly and reliably.
In an article by Hill et al, Obstetrics and Gynecology, 49:236 (February, 1977) it has been suggested that the adaptation of the spring-loaded needle which is currently being used in laparoscopy to induce pneumoperitoneum, could reduce the likelihood of fetal complications. The authors report that virtually every part of the fetus has been struck by needles used for amniocentesis. In an effort to reduce fetal morbidity and mortality, Hill et al adapted a spring-loaded needle for amniocentesis, whereby the sudden movement of an inner trocar would signal penetration into the amniotic sac, thereby obviating further advancement of the needle.
Although improvements in the amniocentesis procedure have been developed and are still being developed, any further improvement for decreasing the failure rate and particularly the risk of fetal complications will be a welcome contribution, especially in view of the fact that the procedure is becoming more widespread in the practice of obstetrics.